Staff at the Edwards Air Force Base in California have used 3D printing to test the Global Hawk, an unmanned surveillance aircraft operated by the U.S. Air Force. By attaching 3D printed nylon attachments to the plane’s wings, staff managed to replicate the effect of icing caused by cloud decks.

Ever since the terrorist attacks of 9/11, after which the U.S. government significantly increased its global surveillance operations, the Northrop Grumman RQ-4 Global Hawk has been in constant military service. The unmanned surveillance vehicle, initially designed by Ryan Aeronautical, can survey around 40,000 square miles of terrain a day using a high-resolution radar and long-range EO/IR sensors. And despite a number of failures during its early years of deployment, the Global Hawk has become a trusted vehicle thanks to its high-altitude capabilities and long loiter times.

As an unmanned aircraft, the Global Hawk is designed to fly much higher than most manned vehicles, and as such needs to be highly resistant to the dangers associated with high-altitude flying. One significant risk to the aircraft is icing, to which it is most vulnerable when traversing cloud decks between 8,000 and 22,000 feet. Icing on the vehicle can add weight and create drag, too much of which could cause total failure.

Engineers of the Global Vigilance Combined Test Force at the Edwards Air Force Base need to test the Global Hawk’s ability to resist icing, but doing so can be tricky: to intentionally induce this icing process, which occurs for about five minutes when ascending and then another five minutes descending, the CTF would need to wait for the right weather conditions and cloud formations, wasting time that the busy team simply doesn’t have. Fortunately, another solution was discovered—one which utilizes 3D printing technology.

To replicate the effect of icing without real ice, the team used a selective laser sintering (SLS) 3D printer to build a number of 3D printed nylon attachments for the Global Hawk which would affect the vehicle in essentially the same way as ice. "SLS allows for complex geometries without support structures and produces parts that are air and water-tight, heat resistant, and strong,” said project engineer Jonny Kim. “This production process effectively saves time and money.”

Models were used to predict the outcome of the 3D printing-assisted tests, but these only gave the CTF a rough idea of how the actual flight would go. As far as the team was concerned, this testing procedure was exploring completely new territory: “This was a first-of-type testing done here at Edwards,” said Lt. Col. Cory Naddy, director of the Global Vigilance CTF. “No other program or CTF has accomplished icing testing in this manner.”

Although the Global Hawk actually flew better than the model-assisted predictions suggested it would, conducting the tests was far from easy. According to Naddy, utilizing the 3D printed attachments posed a significant risk to the wellbeing of the aircraft, but was a choice that had to be made for the benefit of future flights. ”We actually had the aircraft take off with the equivalent of light time icing on the wings and tail — something that no other aircraft would normally do," Naddy explained. "From the moment we lifted off, we were operating in an envelope that was unproven and many risk factors were at play.”

A number of ground tests were carried out before the aircraft took flight. When it finally did so, the CTF flew it with different fuel loads, to test its resistance to the 3D printed mock ice in a range of realistic situations. At first, the Global Hawk was given a medium fuel load, then two successive flights saw the UAV take heavy loads. On each flight, the aircraft was gradually flown at higher altitudes.

According to Maj. Ryan Finlayson, test pilot with the Global Vigilance CTF, these tests could have important—if not immediate—consequences for the Global Hawk: "We've shown that the aircraft is flyable if it picks up a bit of ice," he said.